The present invention relates to a method of casting and a casting machine, more precisely relates to a method of casting and a casting machine, in each of which a molten metal is poured into a cavity of a casting die so as to cast a product having a prescribed shape.
Many kinds of ways of aluminium casting. For example, gravity casting has some advantages: simple casting dies, high quality products, etc.
The casting die of the gravity casting is shown in FIG. 10. In FIG. 10, the casting die 100 is made of a metal. The casting die 100 is a splittable die constituted by a lower die 102a and an upper die 102b. A cavity 104, in which a product having a prescribed shape is cast, is formed by the lower die 102a and the upper die 102b. 
The upper die 102b includes: a metal inlet 106, from which a molten metal, e.g., a molten aluminium, a molten aluminium alloy, is poured into the cavity 104; a feeder head 108 provided between the metal inlet 106 and the cavity 104; and air ventilation holes 110, from which air in the cavity 104 is discharged when the molten metal is poured into the cavity 104.
When the molten aluminium or aluminum alloy is solidified, its volume is reduced about 3% due to shrinkage. The shrinkage of the solidified metal in the cavity occurs a surface sink, etc. in the cast product. In the casting die 100 shown in FIG. 10, a gap, which is formed in the cavity 104 by the shrinkage of the solidified metal, is filled with a part of the molten metal in the feeder head 108, so that the defect, e.g., the surface sink, can be prevented.
Surface tension of the molten aluminium or aluminium alloy is made greater by an oxide film formed on a surface of the molten aluminium or aluminium alloy. Therefore, fluidity and running property of the molten aluminium or aluminium alloy are low, and smoothness of a surface of the product is also low. To solve these disadvantages, inner faces of the feeder head 108 and the cavity 104 of the casting die 100 shown in FIG. 10 are coated with lubricant so as to improve the fluidity and the running property of the molten metal whose surface is covered with the oxide film.
When the product is cast by the casting die 100 shown in FIG. 10, the molten aluminum or aluminium alloy is poured into the metal inlet 106 of the casting die 100. The cavity 104 and the feeder head 108 are filled with the molten aluminium or aluminium alloy with discharging the air from the air ventilation holes 110.
Next, the casting die 100, in which the molten metal has been filled, is cooled so as to solidify the molten metal in the cavity 104. By the solidification of the molten metal in the cavity 104, the solidified metal is shrinked and the gap is formed in the cavity 104, but the gap in the cavity 104 is filled with the molten metal supplied from the feeder head 108.
However, in the conventional method of aluminium casting shown in FIG. 10, the inner faces of the feeder head 108 and the cavity 104 must be coated with the lubricant so as to improve the fluidity and the running property of the molten metal whose surface is covered with the oxide film. But, it is very difficult for inexperienced workers to define coating portions and to uniformly form coating layers. Therefore, surface defects of cast products, e.g., rough surfaces, cannot be avoided.
The inventors of the present invention invented and filed an improved method of aluminum casting (Japanese Patent Application No. 2000-108078), in which aluminium products having good and smooth surfaces can be cast without coating any lubricant.
The improved method will be explained with reference to FIG. 11. Firstly, a magnesium nitride compound (Mg3 N2), which is an example of deoxidizing compounds, is introduced into the cavity 104 of the casting die 100, then the molten aluminium or aluminium alloy is poured therein.
In the improved method, the deoxidizing compound is previously existed in the cavity 104 of the casting die 100, so that the oxide film formed on the surface of the molten aluminium or aluminium alloy can be deoxidized and the surface tension of the molten aluminium or aluminium alloy can be made lower. By deoxidizing or removing the oxide film, the fluidity and the running property of the molten metal can be improved, so that surfaces of the cast products can be smooth and can have good external surfaces.
The feeder head 108 shown in FIG. 10 or 11 is capable of filling the gap, which is formed in the cavity 104 when the solidified metal is shrinked, with the molten metal. Therefore, at least a part of the molten metal in the feeder head 108 must have enough fluidity, even if the molten metal in the cavity 104 is solidified.
Namely, solidifying speed of the molten metal in the feeder head 108 must be lower than that of the molten metal in the cavity 104. Thus, cooling rate of the feeder head must be lower than that of the cavity. To make the difference of the cooling rate, the feeder head 108 is formed into, for example, a pillar shape having broad traverse sectional area. By the pillar-shaped feeder head 108, the molten metal in the feeder head 108 is not easily cooled.
However, the solidified metal in the feeder head 108 is a disused part, so it will be removed from the product. If the solidified metal in the feeder head 108 is reused, it must be molten and energy must be consumed.
Therefore, the pillar-shaped feeder head 108, which has broad traverse sectional area, has greater volume, so yield of casting material must be lower and energy consumption for reuse must be greater.
An object of the present invention is to provide a method of casting, in which volume of a feeder head can be small and cooling rate of the feeder head can be easily made lower than that of a cavity.
Another object of the present invention is to provide a casting machine, which is capable of executing the method of the present invention.
The inventors studied to achieve the objects, and they found that: in their improved method shown in FIG. 11, the deoxidizing compound existed in the cavity 104 of the casting die 100 deoxidizes the oxide film on the surface of the molten meal, so the molten metal has high fluidity on the inner face of the cavity 104, which is not coated with the lubricant; and the cooling rate of the molten metal in the cavity 104, whose inner face is coated with no lubricant, is greater than that of the molten metal in the cavity, whose inner face is coated with the lubricant.
To make a difference of heat insulating between the feeder head 108 and the cavity 104 which are made of the same material, an inner face of the feeder head 108 is coated with the lubricant, and an inner face of the cavity 104 is coated with no lubricant, so that the heat insulating of the feeder head 108 can be made greater than that of the cavity 104.
With this structure, the cooling rate of the feeder head 108 can be lower than that of cavity 104, so that solidification speed of the molten metal in the feeder head 108 can be slower than that of the molten metal in the cavity 104. Then, the inventors reached the present invention.
The method of casting of the present invention is executed in a casting machine including a casting die, in which a feeder head is provided between a metal inlet and a cavity and in which heat insulating of the feeder head is greater than that of the cavity so as to make cooling rate of the feeder head lower than that of the cavity, and said method comprises the steps of:
pouring a molten metal into the cavity;
reacting the molten metal on a deoxidizing compound in the cavity so as to deoxidize an oxide film formed on a surface of the molten metal; and
supplementing the molten metal in the feeder head to the cavity when the molten metal in the cavity is solidified and shrinked.
On the other hand, the casting machine of the present invention comprises a casting die, which includes:
a metal inlet, from which a molten metal is poured into the casting die;
a cavity, in which the molten metal is solidified so as to cast a product; and
a feeder head being provided between the metal inlet and the cavity, in which heat insulating of the feeder head is greater than that of the cavity so as to make cooling rate of the feeder head lower than that of the cavity,
wherein the molten metal is reacted on a deoxidizing compound in the cavity so as to deoxidize an oxide film formed on a surface of the molten metal, and
the molten metal in the feeder head is supplemented to the cavity when the molten metal in the cavity is solidified and shrinked.
In the present invention, the molten metal is reacted on the deoxidizing compound in the cavity of the casting die, and the oxide film formed on the surface of the molten metal can be deoxidized, so that the fluidity of the molten metal can be higher and the product can be cast in the cavity, whose inner face is exposed. Therefore, the lubricant, which improves the fluidity of the molten metal whose surface is covered with the oxide film, is not required.
The lubricant usually has heat insulating, so heat-radiating property of the cavity, whose inner face is coated with the lubricant, is made lower. On the other hand, in the present invention, the molten metal is filled in the cavity, whose inner face is coated with no lubricant, the heat-radiating property can be highly improved. Therefore, the heatradiating property of the cavity of the casting die of the present invention can be easily made high, and the heat insulating of the feeder head can be easily made greater than that of the cavity by coating the inner face of the feeder head with the heat insulating lubricant.
Despite the feeder head is made small, the heat insulating of the feeder head can be greater than that of the cavity, the cooling rate of the feeder head can be made lower than that of the cavity, a difference of the cooling rate between the molten metal in the feeder head and the molten metal in the cavity can be greater, and a difference of solidification speed there between can be made.